Hello,

As there is a standby node, I believe that there should be minimal to no downtime during the upgrade.

Hi,

The standby nodes maintain a copy of the primary node. In the event of a failure or maintenance.
The standby node is immediately promoted to primary and begins serving requests while a replacement standby node is provisioned in the background; so there won’t be any downtime.

For more information on high availability:

https://www.digitalocean.com/docs/databases/#high-availability
https://docs.digitalocean.com/products/databases/mysql/#managed-database-cluster-features

I hope this helps!

Regards,
Rajkishore